The present invention relates generally to tools used in subterranean wells and, in a preferred embodiment thereof, more particularly provides a temporary plug which may be readily dispersed to reestablish flow through a flowbore.
In conventional practice, when an axially extending flow passage or flowbore of a tubing string within a subterranean wellbore must be closed off, it is common to establish a plug within the flowbore to close off the flow of fluids across the plugged off area. For example, retrievable tubing plugs are intended to be easily removed from a flowbore. They are typically run into the tubing on coiled tubing or cable and removed the same way.
If it becomes necessary to reestablish fluid access to that portion of the tubing string closed off by the plug, any other tools present in the flowbore must be removed therefrom before workers can attempt to remove the plug. Removal of the tools and reestablishing of access to the previously closed off portion of the tubing string will usually entail significant cost and rig time. It is, therefore, desirable to develop a plug which may be readily removed or dispersed without either significant expense or rig time.
Some flowbore blocking means have been developed which have a central frangible element that is either pierced or smashed by mechanical means, such as a special wireline tool having a sinker bar and a star bit, or shattered by an increased pressure differential applied at the earth's surface. Also known is a one piece, frangible ceramic sealing element which may be closed to block flow through a flowbore. After use, the element is shattered by impacting with a tooth-faced blind box hammer under force of gravity. Remaining pieces of the ceramic element must then be washed out of the wellbore with completion fluid or the like.
Unfortunately, these designs are not suitable for many customers since elimination of the pieces of the frangible elements, such as by washing them out or by pushing them to the bottom of the well, must be done before the customer can resume operations and is a time-consuming and expensive prospect. Some designs which use a mechanical impact means to destroy the flow blocker require an additional tool run on wireline or coiled tubing to lower and then remove the impact means.
Recently, temporary plugs have been developed which are, in preferred embodiments thereof, composed primarily of a compressed mixture of salt and sand, and which are the subject matter of U.S. Pat. No. 5,479,986 and U.S. application Ser. No. 08/561,754. These types of plugs may be rapidly dispersed, essentially in their entirety, by exposure of the salt and sand mixture to wellbore fluids.
Prior destructible flowbore blocker systems are effective in most situations. However, these systems have generally been configured to block pressurized fluid from one direction, usually downward from the earth's surface, through the flowbore. Some systems, for example, have used hinged, flapper-type valves which pivot closed to block flow through the flowbore. Flow is then reestablished by increasing pressure above the valve to cause destruction of a frangible portion of the valve.
Flapper-type valves are also known in which the frangible portion is destroyed mechanically by, for example, dropping a bar or impacting the frangible portion with another tool. Usually, if significant fluid pressure is applied to these valves from opposite the direction they are intended to block flow from, the valve will open and flow will occur axially through the valve.
Another known plug assembly includes a plug member which has a frangible portion that is shaped in an arcuate fashion such that one side of the plug member presents a convex surface and another side presents a concave surface. So configured, the plug member is significantly more resistant to pressure from its convex side than its concave side. Application of a significant fluid pressure differential from the concave side will likely cause the plug member to be destroyed. As a result, the plug member is, from a practical standpoint, capable of blocking fluid pressure from only a single direction.
From the foregoing, it can be seen that it would be quite desirable to provide a plug which is relatively inexpensive to manufacture, is capable of resisting pressure applied thereto from both axial directions (i.e., is "bidirectional"), and is capable of being dispersed so that no significant restriction or debris remains in the flowbore (i.e., is "disappearing"). It is accordingly an object of the present invention to provide such a bidirectional disappearing plug.